The present invention relates to casting by the centrifugal casting method one or several cemented carbide rings into cast alloys based on iron, preferably cast iron. The resulting product is a composite roll, made in one piece only, with a metallurgical bond between cemented carbide and cast iron.
Composite rolls with cemented carbide for hot or cold rolling comprise one or several cemented carbide rings attached to a (driving) spindle by various couplings and locking devices. One method of transmitting the torque from the driving spindle to the cemented carbide roll is to use keyways or other driving grooves or driving lugs, made integral with the cemented carbide ring. However, cemented carbide is a brittle material with limited tensile strength and special notch sensitivity such as in inner corners in above-mentioned devices for the torque transmission. Methods based on such conventional joints have proved unsatisfactory. Another method for the torque transmission is by means of frictional forces at the bore surface of the cemented carbide ring. However, the radial force on this surface gives rise to tangential tensile stresses in the cemented carbide ring with a maximum at its inner diameter. These tensile stresses are added to other tensile stresses, generated when the roll is in use.
By casting a casing of an iron alloy onto a cemented carbide ring, a composite roll for hot or cold rolling is obtained in which required devices for torque transmission may be located to the cast iron. See, for example, Swedish patent no. 7100170-5, publication number 371114. However, this is not an entirely simple problem. Due to the fact that during cooling the casing shrinks more than the cemented carbide ring, inwardly directed forces on the cemented carbide ring are produced, giving rise to axially directed tensile stresses on the outer surface of the cemented carbide ring, which are acting perpendicularly to microcracks generated in the roll surface during rolling. Under the influence of these tensile stresses, the microcracks propagate in depth, which may cause roll breakage or need for excessive dressing amount, limiting the total rolling capacity of the roll.
A solution to this problem is described in U.S. Pat. No. 5,044,056, the disclosure of which is hereby incorporated by reference. According to that patent, one or several cemented carbide rings are cast into a casing of an iron alloy. The cast alloy comprises an essentially graphitic cast iron which after the casting contains residual austenite, preferably 15-20% by weight, which by one or more subsequent heat treatment steps totally or partly is transformed under volume increase to mainly bainite, with the differential shrinkage between the cast iron and the cemented carbide as a result being at least partly eliminated by the transformation of austenite to bainite during cooling after casting.
The method according to U.S. Pat. No. 5,044,056 permits the manufacture of roll rings which by couplings and locking devices on a driving spindle are assembled to a complete roll. Even if the necessary couplings for the torque transmission are made in the cast iron part, there are dimensional limitations and thereby a limit for the transmittable torque. Further, the wear resistant surface (the barrel) of the rolls is dimensionally restricted. According to the method of that patent or other existing systems of cemented carbide rolls operating by way of some kind of torque transmitting couplings, one is not totally free to place cemented carbide rings on the barrel because of space requiring couplings and locking devices. Thereby, the surface to be used for rolling becomes substantially restricted.
Centrifugal casting is a well-known technique for manufacture of usually rotary symmetric bodies. The technique utilizes the centrifugal force, accomplished by a rotating cylindrical mold, to sling the molten metal against the walls of the mold, where it solidifies under pressure to desired shape. The technique is commonly used for manufacture of tubes, linings, bushes, etc., but is also used for manufacture of rolls. For example, in Swedish patent application no. 8603987-2, a composite roll for hot or cold rolling mills is disclosed, having a centrifugally cast outer wear resistant layer or casing of a high alloyed cast steel and a core of a vermicular cast iron, ductile iron or simple cast steel.